The invention relates to a method for essentially simultaneously forming, from a single series of ink droplets and using an ink-jet printer, several pixels on a substrate, wherein groups of ink droplets from the series are allocated to the pixels, and wherein, depending on the number of ink droplets required for a pixel, the ink droplets are subsequently either sent to the pixel concerned or intercepted. Such a method is known in practice.
Modern ink-jet printers are often designed for printing several image lines essentially simultaneously during a printing movement, in other words, during a movement of the print head relative to the substrate. For this purpose, several pixels situated on different image lines are in each case printed essentially simultaneously by distributing a series of ink droplets over said pixels. In this case the number of ink droplets from which each pixel is composed can vary, depending on the image to be printed. However, within the series of ink droplets an (in principle, fixed) assignment is made beforehand, in which a group of ink droplets from the series is allocated in each case to the different lines, and thus to the different pixels. For each set of pixels to be formed simultaneously, at least one series of ink droplets in which a number of ink droplets is always reserved for a specific pixel is therefore available. In addition, several series of, for example, different colours are provided.
Since each pixel can be composed of a variable number of ink droplets, or at any rate of a variable number of a specific series (colour), only some of the number of ink droplets allocated to a pixel can be sent to the substrate. For that purpose, the ink droplets not required are intercepted. Only part of each series of ink droplets therefore generally reaches the substrate.
In a method of the type mentioned in the preamble, the problem occurs that the ink droplets influence each other on their way to the substrate. Not only can air whirls influence the paths of the ink droplets, but electric charges of the ink droplets can also repel each other. This means that it is possible that the ink droplets may not reach the envisaged place on the substrate, with the result that the pixels are not formed well and the image quality is adversely affected.
Although electrically charged ink droplets with the same polarity repel each other, ink droplets which, in spite of that, go very close to each other coalesce, with the result that the correct image structure may likewise be distorted.
In order to solve these problems, it was proposed earlier that each set of pixels (situated on different image lines) be formed not in succession, but staggered (interlacing). That reduces the coalescence of ink droplets, but the ink droplets still influence each other.
An alternative form of interlacing is described in Japanese Patent Application JP 03-055259. In that application successive pixels are divided alternately into a first and a second group, which groups are printed in different cycles. This means that several printing cycles are always necessary, so that the necessary printing time increases.
It has also been proposed that every other droplet from each series of ink droplets be intercepted. Although this may greatly reduce the extent to which the droplets influence each other, the output of the ink-jet printer is halved.
European Patent Application EP 0 036 788 discloses an ink-jet printer for essentially simultaneously printing several image lines in a grid, in which twenty-four of every fifty-six ink droplets are intercepted in accordance with a predetermined pattern. The output of this known ink-jet printer is thus at most approximately 57%. In addition, only a single ink droplet is allocated to each pixel, which greatly restricts the printing possibilities and the printing quality to be achieved.
The object of the invention is to eliminate the abovementioned disadvantages of the prior art and to provide a method for forming pixels on a substrate which allows the ink droplets to influence each other as little as possible, while giving the greatest possible output. To this end, a method of the type mentioned in the preamble is characterized in that the ink droplets which are sent to a pixel are distributed essentially homogeneously over the series.
The extent to which the ink droplets influence each other is minimized by distributing the ink droplets sent to a pixel on the substrate homogeneously over the series as far as possible. Owing to the fact that the predetermined intercepting of, for example, every other ink droplet is no longer necessary in most cases, the output of the ink-jet printer is considerably improved. While in the ink-jet printer according to the prior art a predetermined number of ink droplets is always intercepted, irrespective of the image to be printed, in an ink-jet printer according to the invention in principle ink droplets are intercepted only in order to achieve a certain grey tone.
The invention further provides an ink-jet printer in which the abovementioned method is applied.